Your OS and Games on the same disc has been a standard "not good" idea for many years now even back since IDE days, due to HDDs having to access data from different points on the hardware it will slow down things, and obviously with games they get updated... especially mostly on steam :P"

This part isn't particularly clear for me. I've got a 256GB Crucial M4. My OS is installed on it but I want to have a couple of games running off it too (namely Skyrim and BF3), so other than lifespan, which I'm not bothered about, what is the downside of running an OS and a couple of games off a single SSD?

For the simple reason that while gaming the game has to access APIs contained in other files within the windows folder structure, i.e. on another part of the disc (or in SSD types part of the memory), which increases load time and wear and tear.

But for SSD its more the load time increase and the constant back and forward of data reading. On a disc based storage device that would potentially ruin the reading head and loosen it if it does that for a lot over a long time. In SSD it can generate heat as it jumps from chip to chip to read data increasing the temperature on the board, and since SSD drives are mostly small 2.5" enclosed chipsets the heat builds up quickly (much quicker than just normal use)

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HDD speeds aren't really determined by what is on the drive, but how its put on the drive.

If a game is installed and files are fragmented and in different parts of the drive that will cause a slow down of reading as the needle has to jump to different areas of a drive.

Hence why its best to at least defragment a HDD once per month.

I am using a 2TB harddisk > SSD cache system, meaningthe primary 2 TB harddisk is cached by a SSD.What i noticed however is after i defragmented the HD some month ago, I did 1 month later a analyze of the defragment level,and weird enough for all my "partitions" on the HD it said 0 % fragmented, also i dont use scheduled defrag.

I am not sure if this HD > SSD caching keeps or kept the HD defragmented, but somehow it looks like it helps to keep the HD defragged.